1. Field of the Invention
The present invention relates to optical weapon sight reticles, and more particularly, to such reticles which exhibiting minimal retroflection therefrom.
2. Background Art
Optical sighting and observation devices used in military applications, such as a tank gun sight, frequently possess “cat's eye” reflection or “retro-reflection” characteristics. That is, light incident on the device is reflected from one or more lens, mirror, prism or other optical surfaces and redirected out of the device in exactly the direction from which it came. Thus, particularly in a night environment, the sighting device may effectively act as a beacon, transmitting light back to an enemy indicating the position of the optical sight and its user. The adverse consequences of revealing the user's position in this manner are readily apparent. Hence, a formidable objective in designing and constructing military sighting devices is to eliminate any retro-reflection characteristics.
Although all optical surfaces within a given optical system may contribute to retro-reflection, some optical elements contribute more prominently to retro-reflection than other optical elements. The optical element addressed herein is the optical reticle. The optical reticle is the cross-hair optical element provided in the focal plane as a sighting reference. The optical reticle usually consists of a plano-plano polished glass element with fine lines or grooves etched therein and filled with a diffusely reflecting solid such as titanium dioxide. When the reticle is illuminated from the side, preferably with an appropriate red illumination when used at night, the solid material stands out and serves as a highly visible reference for the user. Unfortunately, however, the reticle is a large contributor to the retro-reflection and, hence, the optical “signature” of any given optical sighting device. This is due to the fact that light rays collected by the sighting device and incident upon the polished surface of the reticle are redirected back through the optical system toward the source, i.e. the enemy, without any reduction in intensity due to angular dispersion effects.
One method used to minimize such retro-reflective signature from military reticles is commonly known as the sandwich cemented reticle, which involves cementing a glass cover plate to the reticle. Therefore, sandwich cemented reticles involve three optical media: the reticle substrate, the adjacent cover plate, and an optical cement between two reticle substrate and cover plate. Thus, the indices of refraction for all three optical media must be identical in order for the reflectance R to approach zero. Otherwise, the sum of the reflections at the cover plate to cement interface and the cement to reticle interface will be appreciable. The solution to this matching problem has proven elusive. Indeed, aging, humidity, temperature, infra-red exposure and numerous other potential problems may affect optical stability and over time and destroy what had been believed to be ideal optical matching conditions. In particular, such effects over time can alter the properties, including index of refraction of the optical cement. Furthermore, it is difficult to obtain appropriate optical quality glass and cement having identical indices of refraction. Some optical cements are specially prepared to match reticle substrate and cover plate refractive indices, but such cements do not provide sufficient refractive index control to yield the desired degree of reflection suppression.
A second method, disclosed in U.S. Pat. No. 4,981,331, discloses also use of two flat lens surfaces similar to the sandwich cemented reticle; however these surfaces are highly polished and adhere to each other without any cement therebetween, i.e. a polished flat optical cover glass contacted to a polished flat reticle substrate. However, the polished flat optical covered glass and polished flat reticle substrate disclosed in U.S. Pat. No. 4,981,331, still emits a relatively significant retro-reflective signature, due in significant part to the size and edge characteristics of the chemically etched reticle, and has other issues, such as physical stability in field conditions—where the combination is subjected to significant shock, variations in altitude/pressure, and extremes of temperature. Plus, this marriage of two flat lens, exhibits a parallax effect along the edge thereof. And finally, the chemical process for etching fine reticle lines is expensive, as are the two highly polished glass lens.
There is a need in the art for a method to create an economical reticle that does not require a complex and expensive construction, that can utilize curved lens glass so as to reduce parallax at the edge thereof, and that will minimize any retro reflective signature.
These and other highly desirable and unusual results are accomplished by the present invention in a compact structure which can achieve a high degree of reflection suppression on a repeatable basis.